The present invention relates generally to semiconductor device packaging, and, more particularly, to a method and structure for selective thermal paste deposition and retention on integrated circuit chip modules.
The removal of heat from electronic components is a problem continuously faced by electronic packaging engineers. As electronic components have become smaller and more densely packed on integrated boards and chips, designers and manufacturers now are faced with the challenge of how to dissipate the heat generated by these components. It is well known than many electronic components, especially semiconductor components such as transistors and microprocessors, are more prone to failure or malfunction at high temperatures. Thus, the ability to dissipate heat often is a limiting factor on the performance of the component.
Electronic components within integrated circuits have been traditionally cooled via forced or natural convective circulation of air within the housing of the device. In this regard, cooling fins have been provided as an integral part of the component package or as separately attached elements thereto for increasing the surface area of the package exposed to convectively developed air currents. Electric fans have also been employed to increase the volumetric flow rate of air circulated within the housing. For high power circuits (as well as smaller, more densely packed circuits of presently existing designs), however, simple air circulation often has been found to be insufficient to adequately cool the circuit components.
It is also well known that heat dissipation, beyond that which is attainable by simple air circulation, may be effected by the direct mounting of the electronic component to a thermal dissipation member such as a “cold-plate” or other heat sink. The heat sink may be a dedicated, thermally conductive metal plate, or simply the chassis of the device. However, the thermal interface surfaces of an electronic component and associated heat sink are typically irregular, either on a gross or a microscopic scale. When these interfaces surfaces are mated, pockets or void spaces are developed there in-between in which air may become entrapped. These pockets reduce the overall surface area contact within the interface, which, in turn, reduces the efficiency of the heat transfer therethrough. Moreover, as is also well known, air is a relatively poor thermal conductor. Thus, the presence of air pockets within the interface reduces the rate of thermal transfer through the interface.
To improve the efficiency of the heat transfer through the interface, a layer of a thermally conductive material typically is interposed between a heat sink device and electronic component to fill in any surface irregularities and eliminate/reduce air pockets. For example, IBM's ATC 3.8 (advanced thermal compound) is a thermal paste applied to the surface of a chip or protective metal cap of a single chip module (SCM) or multichip module (MCM). The amount of paste volume applied is typically 2 to 3 times the volume of the gap between the chip surface and the pedestal of the protective metal cap. Once the metal cap is pressed onto the top surface of the module during a cap attachment operation, the thermal paste fills the gaps between the chip surface and pedestal of the metal cap for effective thermal management.
However, as a result of the cap attachment process, some volume of the excess thermal paste is typically squeezed out from the chip surface and deposited onto the module surfaces adjacent to the chip. For those module configurations where there are separately mounted passive components (e.g., capacitors and resistors) in close proximity to the chip (and the chip is not underfilled), the excess paste squeezed out from the attachment process may be deposited on the passive components and underneath the chip. Unfortunately, the presence of thermal paste upon certain passive components can degrade the frequency response of analog as well as digital chip modules. In particular, passive components (such as resistors located on the module top surface or signal lines buried in the module substrate) can carry signals in the gigahertz range. If covered by a thermal paste, absorption of the high-frequency signal can take place. This in turn can have a negative effect on the module performance and cause the module not to meet designed electrical specifications.
Accordingly, it would be desirable to be able to implement the application of a thermal paste for integrated circuit chip module in a manner that prevents the paste from spreading to unwanted areas such as on passive components, substrate wiring traces, and beneath the chip(s).